Algal booms low phosphate and nitrate levels in Polluted water?

Algal booms low phosphate and nitrate levels in Polluted water?

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Why does eutrophication (algal booms) take place in polluted water with low phosphate (1mgL-1) and nitrate (0.8mgL -1) concentration?

Nitrates and Phosphates and Algae, Oh My!

Algal blooms, like the one in the photo to the right, occur naturally when nutrient-rich cold water from the seafloor rises to the surface, stimulating rapid reproduction of microscopic algae (phytoplankton). Phytoplankton is essential to the health and productivity of the oceans, as it forms the base of the marine food web, providing sustenance for higher level consumers — namely fish.

Most phytoplankton are too small to be individually seen without a microscope. However, when there is a high enough concentration of phytoplankton, they may appear as a green discoloration of the water due to the presence of chlorophyll within their cells. This is called an algal bloom. Phytoplankton obtain energy through photosynthesis and therefore live in the well-lit surface layers of oceans, seas, and lakes. In addition to light, algae are also dependent on the availability of nutrients such as nitrate, phosphate or silicic acid for growth.

A limited amount of nutrients is available naturally, which generally keeps the growth of algal blooms from getting too large. However, effects of human civilization are trickling their way into the oceans. Runoff from fertilizers applied to agricultural fields, golf courses, and suburban lawns deposition of nitrogen from the atmosphere erosion of soil containing nutrients and discharge from aquaculture facilities and sewage treatment plants are increasing the nutrient content in coastal waters and consequently increasing the size and longevity of algal blooms. This process of nutrient loading is known as eutrophication. Algal blooms can disrupt normal functioning of marine ecosystems, causing a variety of problems such as depletion of the oxygen in the water that fish and shellfish need to survive. Certain species of marine algae produce harmful blooms, known as red or brown tides, which can be toxic to both marine animals and humans.

In Part A of this investigation, you will grow your own algal bloom and test the effects various nutrients and other water qualities have on the occurrence and intensity of algal blooms. In Part B, you will look at two years of satellite data showing chlorophyll concentration in the Sea of Cortés and read an article describing the one-to-one correlation between agricultural runoff and algal blooms in this region. In Part C, you will learn about two potentially deadly consequences of eutrophication — dead zones and harmful algal blooms (HABs).

Nitrates and Their Effect on Water Quality – A Quick Study

How does the presence of Nitrates in our water really effect us?

Does the presence of nitrates affect water quality?

Unlike temperature and dissolved oxygen, the presence of normal levels of nitrates usually does not have a direct effect on aquatic insects or fish. However, excess levels of nitrates in water can create conditions that make it difficult for aquatic insects or fish to survive.

Algae and other plants use nitrates as a source of food. If algae have an unlimited source of nitrates, their growth is unchecked. So, Why is that a problem?

A bay or estuary that has the milky colour of pea soup is showing the result of high concentrations of algae. Large amounts of algae can cause extreme fluctuations in dissolved oxygen. Photosynthesis by algae and other plants can generate oxygen during the day. However, at night, dissolved oxygen may decrease to very low levels as a result of large numbers of oxygen consuming bacteria feeding on dead or decaying algae and other plants.

Eutrophication – “The process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates. These typically promote excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish.

Anoxia – Anoxic Event:Anoxia is a lack of oxygen caused by excessive nutrients in waterways which triggers algae growth. When the plants die and decay, oxygen is stripped from the water, which then turns green or milky white and gives off a strong rotten egg odour. The lack of oxygen is often deadly for invertebrates, fish and shellfish.

Can the presence of nitrates affect human health?

People who use wells as a source of drinking water need to monitor the level of nitrates in their well water. If you drink water that is high in nitrates, it can interfere with the ability of your red blood cells to transport oxygen. Infants who drink water high in nitrates may turn “bluish” and appear to have difficulty in breathing since their bodies are not receiving enough oxygen.

Just like dissolved oxygen, temperature, and pH, the amount of nitrates in water is determined by both natural processes and human intervention. A body of water may be naturally high in nitrates or have elevated nitrate levels as a result of careless human activities.

Why do we need nitrogen? What are the sources of Nitrogen?

Nitrogen i s essential for all living things: animals and plants. Nitrogen forms a part of the proteins and DNA that are found in cells. Animals get nitrogen by eating plants and other animals.

Just like animals, plants require nitrogen to grow and survive. But they do not get nitrogen by consuming proteins like animals do. Plants get nitrogen from water and from the soil. They get nitrogen by absorbing it in the form of nitrates and ammonium. Nitrates are the major source of nitrogen for aquatic plants .

Nitrates are not utilized by aquatic organisms such as fish and aquatic insects, but nitrates are used by aquatic plants.

Where do Nitrates come from?

All aquatic organisms excrete wastes and aquatic plants and organisms eventually die. These activities create ammonia. Some bacteria in the water change this ammonia to produce nitrite which is then converted by other bacteria to nitrate. Nitrates (NO 3- ) are an oxidized form of nitrogen and are formed by combining oxygen and nitrogen.

Nitrates also come from the earth. Soil contains organic matter, which contains nitrogen compounds. Just like the ammonia in water, these nitrogen compounds in the soil are converted by bacteria into nitrates.

Although nitrates occur naturally in soil and water, an excess levels of nitrates can be considered to be a contaminant of ground and surface waters. Most sources of excess nitrates come from human activity. The source of excess nitrates can usually be traced to agricultural activities, human wastes, or industrial pollution .

Nitrogen fertilizers have been applied to yards, fields, golf courses to promote the growth of plants. Rainwater can wash nitrates in the fertilizer into streams and rivers or the nitrates can seep into ground water. This can also occur with animal waste and manure.

In addition to animal waste, untreated human sewage can contribute to nitrate levels in surface and ground water. Leaking or poorly functioning septic systems are a source of such nitrates. City sewage treatment plants treat sewage to make it non-hazardous, but treatment plants still release nitrates into waterways. In addition, industrial plants and agricultural processing operations are potential sources of nitrate pollution.

How do nitrates affect human health?

Nitrate concentrations are monitored in municipal water supplies and foods to prevent exposing people to the potential harmful effects of high levels of nitrates. Nitrates are highly soluble, meaning that they easily dissolve in water. For many people in rural areas, the primary source of drinking water is well water, which may be contaminated with nitrates. Nitrates are colorless and odorless, so their presence cannot be determined without the use of special testing equipment.

Nitrates can interfere with the ability of our red blood cells to carry oxygen. Infants are more at risk of nitrate poisoning than older children or adults. Babies can turn “blue” when there is not enough oxygen being transported by their blood. This “blue baby syndrome” (technically known as methemoglobinemia) is a serious condition that can cause brain damage or death.

How do nitrates affect the health of aquatic animals?

Fish and aquatic insects can be affected indirectly by increased nitrate concentrations in the water.

Basically, any excess nitrate in the water is a source of fertilizer for aquatic plants and algae. In many cases, the amount of nitrate in the water is what limits how much plants and algae can grow. If there is an excess level of nitrates, plants and algae will grow excessively.

Excess plants in a body of water can create many problems. An excess in the growth of plants and algae create an unstable amount of dissolved oxygen. During the day, there will be usually be high levels of dissolved oxygen, and at night the levels of oxygen can decrease dramatically.

  • This will create stressful conditions for fish. If they are stressed for a significant part of the day, they will not behave normally or reproduce. If the conditions persist for a long period of time, the stressed fish species may choose to leave that area or die off.
  • Excess algae or plant growth is also unsightly. If you’ve ever been to a beach where mats of rotting algae wash up on shore or the bottom of the lake is teaming with weeds, it’s probably because excess nitrates are available for plant growth.

Excess plants and algae will also create conditions where organic matter accumulates. High densities of algae will create a condition where sunlight cannot reach very far into the water. Since plants and algae require some sunlight, plants and algae not receiving sunlight will die off. These dead plant materials will settle to the bottom of the water and bacteria that feed on decaying organic material will greatly increase in numbers. These bacteria will consume oxygen and, therefore, the level of dissolved oxygen in this water will fall to levels that are too low for many aquatic insects and fish to survive. Also, this can cause extreme changes in habitat. Fish that need gravel or sand for spawning may find nothing but mats of vegetation and muck so will be unable to produce offspring.

Information by: Partnership For Environmental Education and Rural Health

PEI News Items on Wheatley River Anoxic Events.

Journal of Introductory Biology Investigations

We observed from other studies that in bodies of water that have high nitrate levels, the water seemed to be more polluted and have more algae growth. This lead us to the question of, does nitrate promote the growth of algae? We hypothesized that the higher the nitrate levels the more algae will be produced because nitrates are essential for the production of proteins. In order to test our hypothesis we placed different levels of nitrate, 2 ML, 4ML, 6ML, in a photo bioreactor along with 4ml of algae in each photobioreactor. It was then placed under a light for a week then observed for a change in algae cells. We saw a positive effect between the amount of nitrates added and algae growth. Algae growth was maximum at the highest amount of nitrates added. Our results showed that when nitrate was added, there was an increase of algae growth compared to when no nitrate was added. Thus, supporting that the nutrient nitrate does play a role in algae growth.

Science Center Objects

Like people, plants need nutrients, but too much of a good thing can be a problem. Nutrients, such as nitrogen and phosphorus, occur naturally, but most of the nutrients in our waterways come from human activities and sources—fertilizers, wastewater, automobile exhaust, animal waste. The USGS investigates the source, transport, and fate of nutrients and their impacts on the world around us.

Nutrients are essential for plant growth, but the overabundance of nutrients in water can have many harmful health and environmental effects. An overabundance of nutrients—primarily nitrogen and phosphorus—in water starts a process called eutrophication. Algae feed on the nutrients, growing, spreading, and turning the water green. Algae blooms can smell bad, block sunlight, and even release toxins in some cases. When the algae die, they are decomposed by bacteria—this process consumes the oxygen dissolved in the water and needed by fish and other aquatic life to "breathe". If enough oxygen is removed, the water can become hypoxic, where there is not enough oxygen to sustain life, creating a "dead zone".

A scientist collects water-quality sample to better understand the role of nutrients in the overabundance of duckweed and algae.

Too much nitrogen and phosphorus in water can lead to an overgrowth of free-floating plants such as duckweed and filamentous algae, resulting in dense layers of scum on the surface of the water. This can damage aquatic plants, fish, and other lake organisms by depriving them of the oxygen and sunlight they need to survive. (Credit: James Fischer)


Nutrients are chemical elements found in the food that plants and animals need to grow and survive. Although there are many kinds of nutrients, two of the most important and abundant are nitrogen and phosphorus. Nitrogen and phosphorus occur in a variety of forms, or species, and the species present can change as they move between the air, water, and soil.

  • AMMONIA (NH3) and AMMONIUM (NH4 + ) are among the primary forms of nitrogen in natural waters. Ammonia can be toxic to fish. It is also soluble in water and relatively unstable in most environments. Ammonia is easily transformed into nitrate (NO3 - ) in waters that contain sufficient dissolved oxygen or into nitrogen gas in waters that have no dissolved oxygen.
  • NITRATE (NO3 - ) is another primary form of nitrogen in lakes and streams. Nitrate is verysoluble in water and is stable over a wide range of environmental conditions. It is readily transported in groundwater and streams. An excessive amount of nitrate in drinking water can cause health problems.
  • PHOSPHATES (containing PO4 3− ) are the most common form of phosphorus in natural waters. Phosphates are only moderately soluble and, compared to nitrate, are not very mobile in soils and groundwater. Phosphates tend to remain attached to soil particles, but erosion can transport considerable amounts of phosphate to streams and lakes.

Learn more about nutrients in our Nation's surface water and groundwater.
USGS Circular 1350


Eutrophication is a natural process that results from accumulation of nutrients in lakes or other bodies of water. Algae that feed on nutrients grow into unsightly scum on the water surface, decreasing recreational value and clogging water-intake pipes. Decaying mats of dead algae can produce foul tastes and odors in the water their decay by bacteria consumes dissolved oxygen from the water, sometimes causing fish kills. Human activities can accelerate eutrophication by increasing the rate at which nutrients enter the water. Algal growth is usually limited by the available supply of either phosphate or nitrate, and we say that a water body is nitrogen limited if the ratio of nitrogen species to phosphorus species (N:P) is low, or is phosphorus limited if N:P is high.

Harmful algal blooms (HABs) are can be caused by many different types of algae in freshwater ecosystems, and can be triggered by nutrient enrichment. The most frequent and severe blooms typically are caused by cyanobacteria, the only known freshwater algae with the potential for production of toxins potent enough to harm human health. CyanoHABs can threaten human and aquatic ecosystem health. Economic damages related to cyanoHABs include the loss of recreational revenue, decreased property values, and increased drinking-water treatment costs.

Harmful algal blooms turn water in Milford Lake, Kansas, emerald green. (Credit: Jennifer Graham, USGS)


The USGS works extensively across the country on a variety of aspects related to nutrients and eutrophication. Explore the related projects tab for some examples or click the links below.

4. Soil Loss

Due to the sheer quantity of nutrients used as fertilizers on farms, much of it does not end up in the crops. Instead, the nutrients leach out of the soil, into the groundwater, and create runoff. Nutrient-rich soils are carried into waterways through rainwater. This occurs even more so if the soil is eroded, which it often is in agricultural practices. In areas of natural vegetation, like the rainforest for example, soil is held in place through a variety of botanic roots. Alternately, if the land is ploughed the soil becomes dry and loose, and is easily swept away by forces of wind or water (“Soil Health Management”).

Iowa scientists in the environmental working group, Losing Ground, state that the average soil loss from natural processes is 5 tons per acre per year. Yet Iowa farmland was actually measured to lose an average of 100 tons per acre per year, meaning there is runoff from farms that is actually exceeding this number. A portion of the loss can be attributed to the lack of a buffer zone between cropland and waterways without an area of trees or grass between crop fields and streams, farmers are able to cover more land with crops and produce higher yields. Fortunately research has proved that better soil management techniques can significantly reduce excess runoff, still many of these techniques remain poorly exploited (Zeland 2014)

Cut phosphorus to reduce algae blooms, say scientists

Algal bloom in Manitoba's Lake Killarney. Credit: Diane Orihel

Several prominent Canadian and American scientists are urging governments around the world to focus on controlling phosphorus to decrease the frequency and intensity of algal blooms in freshwaters. Their recommendation follows this week's publication of a feature article in a leading environmental science journal, Environmental Science & Technology. The article reviews the results of whole-lake studies where phosphorus, nitrogen or both elements had been controlled.

"Thirty seven long-term, whole-lake studies conducted in nine countries in Europe and North America showed that controlling a single element, phosphorus, reduced algal blooms in lakes," says David Schindler, professor emeritus at the University of Alberta and the study's lead author. "Studies of controlling nitrogen, either alone or with phosphorus, showed no discernible effect on algal blooms. Lakes where phosphorus reductions successfully reduced algal blooms ranged in size from small ponds to Lake Superior, in a wide range of climatic and geological settings."

In the 1970s, phosphorus inputs to lakes were regulated in Europe and North America in order to halt or reverse the proliferation of algal blooms and related changes in lakes, collectively termed eutrophication. Eutrophication is one of the leading causes of freshwater pollution. The problem costs an estimated $2.2 billion per year in the U.S. alone and is the leading cause of drinking water problems in most developing countries.

However, in the past 10 years, some scientists have argued that controlling phosphorus alone is not enough, and that nitrogen inputs must also be reduced. In response, the European Union has required nitrogen removal as well as phosphorus from sewage effluents, and in 2011 the U.S. EPA announced that it would be "partnering" with states to control both phosphorus and nitrogen. In New Zealand a nitrogen loading cap has been imposed on the watershed of its largest lake, Lake Taupo, without any defined phosphorus loading limit.

Algal bloom in Lake Ontario. Credit: Diane Orihel

According to this new study, reducing nitrogen as part of these control efforts won't actually help the eutrophication problem. "The recommendations of some scientists for nitrogen control are based on correlations, or on the results of short term assays where nutrients are added to small containers of algae. These poorly represent the long-term responses of whole lakes, especially when nutrients are removed, rather than added," says Schindler.

In the sole long-term study where input of nitrogen alone was reduced, the size of algal blooms did not decrease. Instead, the proportion of cyanobacteria increased. Cyanobacteria, often referred to as "blue-green algae" are the "public face of the eutrophication problem," says Steve Carpenter, director of the University of Wisconsin-Madison's Center for Limnology and a contributing author on the paper. Cyanobacteria can flourish even when nitrogen in the water column is reduced because they can "fix" or take in nitrogen from the atmosphere. And that's a problem, according to Carpenter, because some species produce toxins that pose human health concerns, like the cyanobacteria bloom in Lake Erie that caused the shutdown of Toledo's water supply in 2014. What's more, many of the species float and form the scums that accumulate, rot on beaches, and cause fish kills.

In the study, in lakes where nitrogen and phosphorus were both decreased, the decline in algae correlated only with declining phosphorus, while excess nitrogen was either denitrified or accumulated in dissolved form.

"In many ways, our conclusions are good news," says co-author Diane Orihel, Banting and Liber Ero Fellow at the University of Ottawa and UAlberta alumna. Controlling inputs of phosphorus is much less costly than controlling nitrogen, and estimated costs of reducing both elements range from four to eight times higher than reducing phosphorus alone. While nitrogen removal is not necessary to control algal blooms in lakes, it is causing problems with soil acidification and groundwater pollution in some areas. Adds Orihel, "There are many reasons to control nitrogen pollution, but freshwater eutrophication is not one of them."

Algal Blooms

Algal blooms in rivers, creeks and lakes are an increasing occurrence in California, threatening human health and safety as well as pets. Exposure to toxic blue-green algae, also known as cyanobacteria, can cause eye irritation, allergic skin rash, mouth ulcers, vomiting, diarrhea and cold- and flu-like symptoms. Young children are most likely to be affected by harmful algal blooms because of their small body size and tendency to play in the water for longer periods. Dogs also are susceptible because they tend to drink while in the water and lick their fur afterward.

Blue-green algal blooms occur in California during the summer months because hot temperatures combined with low water levels stimulate growth. Algal blooms are aided by wet winters that deposit nutrient-rich sediment into water bodies.

What are algae?

Organisms that photosynthesize but lack the formal water circulation structure of land plants are placed in the broad category of “algae.” These can grow in either freshwater or saltwater environments, existing as single cells (often congregating in colonies) or multicellular organisms such as giant kelp.

Algae are so prolific that their photosynthesis is responsible for producing approximately half of the planet’s oxygen, making them essential to our survival. Old algae deposits also make up a portion of petroleum products, and living or recently dead algae serve as both food and shelter for innumerable species.

If there are excess nutrients present in their environment, especially nitrogen and phosphorus, algae populations grow at accelerated rates. These sudden population surges are referred to as “algal blooms.” Algal blooms can pose a health risk to people and pets.

How are algae dangerous?

Algal blooms often occur faster than their ecosystem can compensate for. Although algae produce oxygen to grow, when they die, they remove oxygen from their environment in order to decay.

Additionally, at night algae undergo respiration in order to develop, a process requiring still more oxygen. Thus, especially in large masses, algae remove much of the finite amount of oxygen in the water.

Fish, bacteria, and even plants require certain amounts of oxygen to live, so algal blooms of even nontoxic species can cause fish kills and otherwise disrupt biodiversity.

Beyond their effects from overpopulating bodies of water, some algae species themselves are toxic. Several species of blue-green algae in particular are notorious for releasing dangerous neurotoxins. Despite their name, these algae are not restricted to blue-green colors and in fact appear as green, yellowish-brown, or red.

Since nontoxic algae are also green, the colors of blooms are not always reliable indicators of toxicity. Because red tides are unmistakably a result of toxic algae, harmful blooms in saltwater environments are often referred to as “red tides” (regardless of their pigment).

How can I determine if an algal bloom is harmful?

Freshwater harmful algal blooms are often more difficult to detect than colorful red tides in marine environments, and generally appear green. Nevertheless, these species usually conform to characteristic patterns that can be recognized from the surface. Grouping in parallel streaks or clumped dots usually indicates toxicity, as does the semblance of spilled green paint or thick pea soup. Alternatively, nontoxic algae generally appear as floating rafts, scum, tangled or sprawled hair and thick mats.

How are fish affected?

Some algal blooms produce dangerous levels of the neurotoxin “domoic acid.” While commercial fish are generally under strict-enough regulatory restrictions for safe consumption, recreationally caught fish are unregulated and could introduce dangerous levels of domoic acid. Many fisheries have been temporarily closed, resulting in millions of dollars of lost revenue. The California Department of Fish and Wildlife releases updated fishing restrictions that include safe locations to avoid harmful domoic acid.

The State Water Resources Control Board works with state and local agencies to identify and respond to hazardous algal blooms throughout California. Formed in 2008, the California Harmful Algal Bloom Monitoring and Alert Program includes algal bloom researchers, water quality and shellfish managers, universities, state and local agencies. The program is a coordinated response network for efficient algal bloom mitigation, prediction and prevention and provides updates on current algal blooms.

Water Pollution Overview

Water pollution is the contamination of water by an excess amount of a substance that can cause harm to human beings and/or the ecosystem. The level of water pollution depends on the abundance of the pollutant, the ecological impact of the pollutant, and the use of the water. Pollutants are derived from biological, chemical, or physical processes. Although natural processes such as volcanic eruptions or evaporation sometimes can cause water pollution, most pollution is derived from human, land-based activities (Figure 2). Water pollutants can move through different water reservoirs, as the water carrying them progresses through stages of the water cycle (Figure 3). Water residence time (the average time that a water molecule spends in a water reservoir) is very important to pollution problems because it affects pollution potential. Water in rivers has a relatively short residence time, so pollution usually is there only briefly. Of course, pollution in rivers may simply move to another reservoir, such as the ocean, where it can cause further problems. Groundwater is typically characterized by slow flow and longer residence time, which can make groundwater pollution particularly problematic. Finally, pollution residence time can be much greater than the water residence time because a pollutant may be taken up for a long time within the ecosystem or absorbed onto sediment.

Pollutants enter water supplies from point sources, which are readily identifiable and relatively small locations, or nonpoint sources, which are large and more diffuse areas. Point sources of pollution include animal factory farms (Figure 4) that raise a large number and high density of livestock such as cows, pigs, and chickens. Also included are pipes from factories or sewage treatment plants. Combined sewer systems that have a single set of underground pipes to collect both sewage and storm water runoff from streets for wastewater treatment can be major point sources of pollutants. During heavy rain, storm water runoff may exceed sewer capacity, causing it to back up and spilling untreated sewage directly into surface waters (Figure 5).

Nonpoint sources of pollution include agricultural fields, cities, and abandoned mines. Rainfall runs over the land and through the ground, picking up pollutants such as herbicides, pesticides, and fertilizer from agricultural fields and lawns oil, antifreeze, animal waste, and road salt from urban areas and acid and toxic elements from abandoned mines. Then, this pollution is carried into surface water bodies and groundwater. Nonpoint source pollution, which is the leading cause of water pollution in the U.S., is usually much more difficult and expensive to control than point source pollution because of its low concentration, multiple sources, and much greater volume of water.

What causes algal blooms?

The development and proliferation of algal blooms likely result from a combination of environmental factors including available nutrients, temperature, sunlight, ecosystem disturbance (stable/mixing conditions, turbidity), hydrology (river flow and water storage levels) and the water chemistry (pH, conductivity, salinity, carbon availability…).

However, the combination of factors that trigger and sustain an algal bloom is not well understood at present and it is not possible to attribute algal blooms to any specific factor. READ MORE about the factors that cause algal blooms.

Nutrients promote and support the growth of algae and Cyanobacteria. The eutrophication (nutrient enrichment) of waterways is considered as a major factor. The main nutrients contributing to eutrophication are phosphorus and nitrogen.

In the landscape, runoff and soil erosion from fertilized agricultural areas and lawns, erosion from river banks, river beds, land clearing (deforestation), and sewage effluent are the major sources of phosphorus and nitrogen entering water ways. All of these are considered as external sources.

Internal origin of nutrients comes from the lake/reservoir sediments. Phosphate attaches to sediments. When dissolved oxygen concentration is low in the water (anoxic), sediments release phosphate into the water column. This phenomenon encourages the growth of algae.

Early blue–green algal blooms usually develop during the spring when water temperature is higher and there is increased light. The growth is sustained during the warmer months of the year. Water temperatures above 25°C are optimal for the growth of Cyanobacteria. At these temperatures, blue–green algae have a competitive advantage over other types of algae whose optimal growth temperature is lower (12-15°C).

In temperate regions, blue–green algal blooms generally do not persist through the winter months due to low water temperatures. Higher water temperatures in tropical regions may cause blue–green algal blooms to persist throughout the year.

Blue–green algae populations are diminished when they are exposed to long periods of high light intensity (photo-inhibition) but have optimal growth when intermittently exposed to high light intensities. These conditions are met under the water surface where light environment is fluctuating.

Even under low light conditions, or in turbid water, blue–green algae have higher growth rates than any other group of algae. This ability to adapt to variable light conditions gives cyanobacteria a competitive advantage over other algal species.

Most of blue–green algae prefer stable water conditions with low flows, long retention times, light winds and minimal turbulence other prefer mixing conditions and turbid environments.

Drought, water extraction for irrigation, human and stock consumption and the regulation of rivers by weirs and dams all contribute to decreased flows of water in our river systems. Water moves more slowly or becomes ponded, which encourages the growth of algae.

In water bodies, another consequence of stable conditions is thermal stratification. Thermal stratification occurs when the top layer of the water column becomes warmer and the lower layer remains cooler. When the two layers stop mixing, the upper layer becomes more stable (no wind-induced mixing, convection cells)and summer blooms of buoyant blue-green algae are supported.

When a water body is stratified, bottom waters often become depleted with oxygen (anoxia) which may lead to increased nutrient release from the sediments. Pulses of nutrient from the colder bottom layer may fuel up the algal growth in the top layer.

Turbidity is caused by the presence of suspended particles and organic matter (flocs) in the water column. High turbidity occurs when a lot of water is running through the system (high discharge after a rain event). Low turbidity occurs when there is only a small amount of suspended matter present in the water column. Low turbidity can be due to slow moving or stagnant water that allows suspended articles to settle out of the water column. When turbidity is low, more light can penetrate through the water column. This creates optimal conditions for algal growth. In return, growing algae create a turbid environment.

11 - The Role of Nitrates in the Eutrophication and Acidification of Surface Waters

Nitrogen is an essential nutrient in aquatic ecosystems but when nutrient availability increases eutrophication can result. Chemical changes are paralleled by changes in biological productivity, the composition and diversity of biota and consequently in physical conditions of water bodies. The main concerns about eutrophication are limitations on the water use and increased costs of treatment but health risks can also be associated with algal blooms. Nitrogen is rarely the limiting nutrient in aquatic systems in most temperate regions waters are P limited but N can be limiting in estuarine and marine waters. Atmospheric inputs of pollutant S and N have lead to the acidification of aquatic ecosystems in acid sensitive areas. The chemical changes lead to changes in biota and a reduction in diversity. Sulfate deposition has been the primary driver of acidification but the importance of N is increasing as S deposition decreases. Both acidification and eutrophication can take place naturally recent concerns relate to acceleration of the processes as a result of ‘human’ activities.

Regional patterns of nitrate concentrations in surface waters in the UK show a trend from lower concentrations in the extensively used uplands of the north and west to higher levels in the more densely populated and intensively farmed south and east. Atmospheric deposition can be the main nitrate input to upland waters but inputs from agricultural land and from water treatment are the main sources in the lowlands. A few areas of enhanced concentrations have been identified in the uplands atmospheric inputs have probably saturated the retention capacity of the catchments in these areas. Areas of acidified waters have been identified in the UK. There is no single data source for the definition of UK waters which have been affected by eutrophication but limited surveys have provided ‘snapshots’ of the occurrence of waters with algal bloom problems.

The critical load concept is being applied to calculate the maximum nitrate plus sulfate inputs from the atmosphere that will not produce acidification. Guidelines and models have also been developed to set or determine the maximum nutrient inputs to lakes to prevent eutrophication.

Watch the video: Nutrient Pollution (August 2022).